In case of incorporating electronic parts, etc. into a case main body and cover molded with thermoplastic resin and coupling this case main body with the cover, such a method is known which utilizes a supersonic wave, high frequency electromagnetic induction or a bonding agent.
However, with a junction method utilizing such a supersonic wave and a high frequency electromagnetic induction has the danger of inducing an inferior factor to the incorporated electronic parts, etc. Further, in case of coupling by the bonding agent, it takes a long time until the agent gets hardened, and sometimes the bonding agent oozes out of the matched face which becomes the cause for stains and the outside appearance becomes worsened.
As a means for solving this kind of problem, Japan Unexamined Patent Publication (Kokai) No. S58-59050 and Unexamined Patent Publication (Kokai) No. S62-288029 present the method for catching a heating element based on the electric resistance between the fused case main body and the cover, fusing the thermoplastic resin by this heating and mutually depositing the case and cover to each other. This is an effective junction method without having an influence on the internal parts and without having an influence on the outside appearance because it is a fusion method by instantaneous heat.
However, in any of the cases, so long as an electrode or a resistance heating element in a conductor state is used for supplying the voltage to the resistance heating element, this conductor protrudes to the outside of the case and becomes a trouble point with regard to assembling the case into another box body. Further, in the case of a single case body, it becomes sometimes a detriment as to the outside appearance. For this reason, post-processing becomes necessary as cutting the protruded portion of the electrode and conductor as necessary. Moreover, it can be imagined that some kind of liquid invades between the fusion of the resistance heating element and the thermoplastic resin and exerts an influence on the fusion strength.